With modern integrated circuits operating at ever increasing speeds and at ever decreasing powers, control of the gate stack films has become more important, and more difficult than ever before. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
Although there are many important electrical characteristics that define the proper behavior of a gate, some of the most critical issues are gate threshold voltage shift and gate leakage. Because the effective oxide thickness of gates is so thin, and the power is so low, gate threshold voltage shifting and high leakage are fatal to the proper operation of the integrated circuit. The electrical characteristics of the gate are predominantly set at a point in time that occurs relatively early in the integrated circuit fabrication process. These steps include preparation of the semiconducting substrate, deposition of the gate insulation layer, and other related, early occurring steps.
Unfortunately, it is much later in the process before the important electrical characteristics that are defined by these processes can be directly measured, such as by contact probing. At that point, however, any problems with the earlier processes that may negatively effect gate electrical properties will have effected a large number of integrated circuits in the fabrication line, potentially resulting in the waste of large numbers of misprocessed or otherwise damaged integrated circuits.
Prior art methods of checking for such problems earlier in the fabrication process include optical inspection for cleanliness and thickness measurement. However, with effective gate oxide thicknesses dropping to such low levels, such relatively crude methods of inspection have proven to be inadequate.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.